Hydroformed pipes are widely applied to processing and manufacturing of automobile parts, so as to reduce the weight and increase rigidity. However, pipes with a great expansion rate, such as exhaust pipes and silencers, are produced through spinning or welding, thereby lowering production rate and having poor rigidity.
When a pipe with an inclined expansion is hydroformed using a fixed forming mold in the prior art, severe friction is generated between a metal pipe blank and an upper die of the fixed forming mold, and the metal pipe blank cannot extend smoothly. As a result, the thickness of a wall of the pipe with an inclined expansion is uneven, buckling and wrinkling are generated, and the pipe wall at the protrusion area may break as the wall is excessively thin.